A fuel cell comprises an electrolyte layer and a pair of catalyst carrying electrodes (referred to as catalyst electrode layers) placed on either side of the electrolyte layer, and generates electricity through an electrochemical reaction between fuel fluid such as hydrogen or alcohol and oxidizing fluid such as oxygen or air, which are supplied to the corresponding catalyst electrode layers, with the aid of the catalyst.
FIG. 1 shows a conventional embodiment of a fuel cell assembly. As shown, this fuel cell assembly 1 comprises an electrolyte layer 10, a pair of catalyst electrode layers 11, 12 disposed on either side of the electrolyte layer 10, a pair of diffusion layers 13, 14 disposed on either outer side of the catalyst electrode layers 11, 12, and a pair of separators (or flow distribution plates) 15, 16 disposed on either outer side of the diffusion layers 13, 14. The separators 15, 16 can be formed by etching a silicon substrate, for example, and formed with channels (or recesses) 20 for defining a flow passage for a fuel fluid (e.g., hydrogen gas) or an oxidizing fluid (e.g., oxygen gas) in their surface facing the diffusion layers 13, 14. The diffusion layers 13, 14 are provided to diffuse the fluids evenly over the electrolyte layer 10 as well as to contact the catalyst electrode layers 11, 12 to thereby transmit electric potential of the electrode layers 11, 12 to outside, and typically formed of an electroconductive porous material such as a carbon paper or a carbon cloth. When the separators 15, 16 are made of an electroconductive material or an insulating or high-resistance material covered with an electroconductive film, external electrodes may be attached to the separators 15, 16. Further, in order to prevent undesirable leakage of the fluids, seal members 17, 18 are disposed between the electrolyte layer 10 and the separators 15, 16 so as to surround the diffusion layers 13, 14.
The fuel cell assembly 1 is formed by stacking these component parts and applying a tightening force on them in the stacking direction so that the adjacent component parts are closely pressed to each other. For this purpose, a pair of backing plates 21, 22 are provided on either outer side of the separators 15, 16, a plurality of rods 23 extend through the backing plates 21, 22, and nuts 24 are engaged with threaded ends of the rods 23 so that the rotation of the nuts 24 can produce the pressure in the stacking direction. A required tightening pressure can be varied for different combinations of component parts to be pressed together. However, it should be noted that because the diffusion layers 13, 14 made of a carbon paper/cloth or the like have a rough surface, a particularly large contact pressure is needed between the diffusion layers 13, 14 and the separators 15, 16 in order to prevent the fluids from leaking as they pass through the diffusion layers 13, 14. Also, when the separators 15, 16 are formed of an electroconductive material such as a metal or the surface of the separators 15, 16 is covered with an electroconductive film to thereby allow the voltage of the diffusion layers 13, 14 to be transmitted to outside through the separators 15, 16, it is required to make the diffusion layers 13, 14 and the separators 15, 16 contact each other with a large pressure in order to reduce the contact resistance therebetween.
However, in order for the diffusion layers 13, 14 and the separators 15, 16 to stand the large pressure, they need to have a high mechanical strength, which could lead to a larger component size and/or higher manufacturing cost. Further, in the above structure, the tightening force applied to the separators 15, 16 and the diffusion layers 13, 14 is also imposed on other component parts which may not require such a large tightening pressure, and therefore these component parts also need to have a high mechanical strength. The large tightening force also tends to necessitate a bulky tightening structure (i.e., backing plates 21, rods 23 and nuts 24). These factors can undesirably increase the weight, volume and manufacturing cost of the fuel cell assembly 1.
The electrolyte of the electrolyte layer 10 may consist of a solid polymer electrolyte (SPE). However, the SPE can function as an ion conducting membrane only when impregnated with water, and the SPE when impregnated with water significantly increases its volume. The volume of the SPE can also change depending on the temperature. Such volume increase of the SPE can generate stress inside the fuel cell assembly 1. Therefore, when the externally applied tightening force is large, the pressure applied to the component parts may become excessively high, which can cause a problem such as breaking the seal members 17, 18. Controlling the pressure at a constant level would result in an undesirably complicated operation.
Generally, in the fuel cell assembly 1, it is preferable that the diffusion layers 13, 14 have high electric conductivity to effectively conduct the potential of the catalyst electrode layers 11, 12 to outside. Also, a smaller surface roughness of the diffusion layers 13, 14 is preferred to lower the contact resistance between them and the adjoining component parts (e.g., catalyst electrode layers 11, 12 or separators 15, 16). Further, so long as a favorable diffusion capacity is achieved, thinner diffusion layers 13, 14 are preferred to achieve a smaller (thinner) fuel cell assembly 1. However, in the conventional diffusion layers 13, 14 made of a carbon paper or carbon cloth, there has been a limit to the increase in the conductivity as well as reduction in the thickness and surface roughness.